Sound recording and reproduction



Dec. 28, 1943. F. s. ALBIN SOUND RECORDING AND REPRODUCTION Filed Sept. 6, 1941 2 Sheets-Sheet 1 Escoeosz 7000 CPS.

low Pass HA TER AMPLIFIER AMP; lF/EAZ AMPL 1 runs Can/Team 3000 6'- HS. Use/1.1.4102

AT pgA/EX Dec. 28, 1943. F. G. ALBIN SOUND RECORDING AND REPRODUCTION Filed Sept. 6, 1941 2 Sheets-Sheet 2 ill/g2 I f II 7//////// d Patented Dec. 28, 1943 SOUND RECORDING AND BEPRODIlCTION Frederick G. Albin, Los Angeles, Calif., assignor to Radio Corporation of America, a corporation of Delaware Application September 6, 1941, Serial No. 409,802

15 Claims. (01. ire-100.3)

This invention relates to sound recording and reproducing apparatus and particularly to a system wherein wave form distortion, caused by overshooting the maximum limit of the recording medium or non-linearity from other causes such as improper processing, non-uniform illumination across the track, dirt, scratches, etc., is substantially eliminated.

It is well known that in the recording of sound on film or discs there are definite limits above and below which the recorded sounds may be either entirely lost or so badly distorted as to be unusable. In the case of sound tracks on motion picture film, the maximum recordable amplitude is controlled bythe width of the sound track, which determines the ratio between complete opaqueness and film transparency. In the case of disc or magnetic recording, the upper range limit is controlled by the width or depth of the grooves or the amount of magnetism obtainable on the magnetic record, respectively. While it is always possible to raise the lower amplitude level above reproduction in cases where the sound track limit has been exceeded, the system also providing an automatic level control simultaneously therewith. Another result obtained is the eliminating of plops 0r crackling caused by comparatively large dirt particles or scratches on certain portions of the sound track area. The invention also restores portions of wave forms destroyed by film of the signal if eliminated by scratches or by overthe noiselevel toobtain a record of the low amplitude sounds, it is not possible to increase the area or mass of a recording medium.

In the art of motion picture sound recording, it is well known that in most instances the sound range to be recorded exceeds that ,of the film, and it has become necessary to. compress the sound range to within the film limits. This is generally accomplished by a mixer who, by observing a volume control indicator, reduces the input level to the recorder to within the film's upper limit. It is also accomplished automatically by electronic compressors well known in the art.

- Although the use oia manual mixer or automatic compressor system is satisfactory in most instances, it frequently occurs that unexpected increases in sound do exceed the upper film limit, and overshooting occurs. In the case of film, this means that the light beam of a variable area recording system moves beyond the edge of the ound track recording area, and the peak portions of the modulations thereby extend beyond the track area and are-not recorded. In the case of variable density recording, the intensity or quanta of the light beam goes beyond the limits of the film characteristic and the variations of th light are thereby lost, similar to the loss of detail in a picture when a portion thereof is in dark shadow or overexposed. Also, clipping due to noise reduction action is always an unavoidable source of distortion.

The present invention is directed to a method of shooting of the limits of the sound recording medium.

For instance, Sacia Patent No. 1,623,756 and Round Patent No. 1,925,608 disclose a two-track pulse is recorded with the signal to, operate switches for different purposes, while copendin application of Relskind Serial No. 326,912, filed March 30, 1940, discloses a pilot frequency system for controlling one or more control signals. In

all of the prior disclosures, however, ther is no method of or system for using a high frequency pilot current to restore missing portions of a signal lost by lack of available recording medium. Also, these prior systems are not suitable for this use without modification in accordance with the present invention, the latter, however, being capable of accomplishing the results of the prior systems.

The principal object of the present invention, therefore, is to facilitate recording and reproduction of sound.

Another object of the invention is to increase the recording range of a given sound recording medium. I

A further object of the invention is to increase the upper recording limit of any given sound recording medium.

and system for restoring the wave formdurlng 58 A further object of the invention is to provide a system of recording and reproduction wherein lost portions of a sound recording arerestored.

A further object of the invention is to provide a method of and system for restoring mutilated, sound tracks to their original wave forms and to eliminate noises caused by dirt, abrasions and scratches on the sound track.

A further object of the invention is to provide a system of recording and reproduction wherein the recorded level is maintained constant and embodying the invention;

Fig. 2 is a block diagram of a sound film reproducer embodying the invention;

Fig. 3 is a schematic diagram of of the reproducer shown in Fig. 2; and

Figs. 4, 5 and 6 are graphs showing theoperating of the invention.

Referring now to Fig. system in accordance with the invention is represented by a microphone 5 feeding a preamplifier 6 which is connected to a mixer 1 of any well known type wherein the volume of the signal is controlled either manually or by an automatic volume compressor. The output of the mixer is connected to an amplifier 9, the output of which is filtered through a 7000-cycle low-pass filter I0 the portion l, a 'sound recording before impression upon'a recorder ll of any The above elements comprise the of the standard type. usual type of sound recorder regardless type of sound track being made.

During the operation of the recording system, there is simultaneously impressed upon the recorder il a 9000-cycle frequency current from an oscillator l3, the amplitude of' the current being controlled at H. The amplitude of the constant frequency 9000-cycle current depends upon the maximum amount of overshooting to be expected in any given recording, although once adjusted to a certain amplitude, this amplitude remains constant during the recording operation unless varied for certain purposes, as. will be explained hereinafter. The type of track impressed upon the recorder is shown in Fig. 4 wherein the signal current is represented by the sine wave a; and the 9000-cycle oscillator current, by the sine wave b. Although the signal current is represented by the sine wave for the sake of clarity, it is understood, of course, that this signal wave will be of'the form representing the sound source.

Referring now to Fig. 2, in which the reproducer system is shown in block diagram, a. standard scanning system comprising a lamp IS, a lens II, a lens l8 and a photocell i9 is shown in conjunction with the film 2| bearing a. sound track 22. The output of the cell i9 is fed into a photocell amplifier 24, then impressed on a variable gain amplifier 25, the output of which is filtered through a TODD-cycle low-pass filter 26, after which it is amplified by power amplifier 2land impressed upon a loudspeaker system 28. The above system just described is the usual type of sound reproducing system with or without the variable gain amplifier. In accordance with the invention, however, there is also connected to the output of the variable gain amplifier 25 an amplifier 30 feeding a. QOOD-cycle bandpass filter 32 which, in turn, feeds a rectifier 33, the output thereof being fed to the gain control electrodes of the variable gain amplifier 25. Thus, it will be observed that the 7000-cycle filter eliminates from the speaker 28 the 9000-cycle control current, while the QOOO-cycle filter '32 eliminates 'the signal current from the rectifier 33.

For detailed operation of the elements shown within the dotted line of Fig. 2, reference is made to Fig. 3 wherein the variable gain amplifier is shown consisting of an input transformer 35, the secondary of which is connected to variable mu electron tubes 36 and 31. The plates of tubes 36 and 31 are connected to the primary of an output transformer 39, the secondary of' which is connected to the 7000-cycle low-pass filter 26 consisting of inductances 4| and condensers 42. An anode and screen grid potential source 44 is shown for the tubes 36 and 31, while a grid biasing potential source 45 is shown connected in series with a grid biasing resistance 46.

Connected to the output circuit of the tubes 36 and 31 is a push-pull amplifier circuit comprising tubes 50 and 5i, these tubes being connected through coupling condensers 52 and resistance 54 with a biasing battery 55. The output circuit-of the tubes 50 and 5| is connected to a rectifier 56 through a 9000-cycle bandpass filter comprising series inductances 58 and condensers 59 and shunting circuits comprising inductances 6| and 62 and condensers 63 and 64. Thus, the recorded outputof the QOOO-cycle oscillator is separated from the signal and is impressed upon the rectifier '56, the output of. which is directly impressed upon the grid biasing resistor 46. The operation of the system will now be described in relationship to the graphs of Figs. 4, 5 and 6.

. To explain the operation of the system, variable area wave forms are used as illustrations, one type of such a signal being shown in Fig. 4 wherein the signal envelope a is shown combined with the control frequency current b. The film limit or edge of the sound track area is shown in Fig. 4 by the broken line 0, and with the amplitudes of the signals and a film limit as shown, the overshot portion not recorded is shown shaded. Thus, it will be observed that the peaks of the signal a between the points d and e of the cycles shown are not recorded on. the film, these peaks thereby having fiat tops. It will also be observed that certain portions of the cycles of the control frequency b are also eliminated in the particular illustration. Thus, the wave form appearing at the edge of the sound track area over the flat top of the signal wave will include dips caused by the pilot frequency.

Since the. purpose of the present system is to restore the peaks of the signal current a to its original form, this is accomplished by utilizing the amplitude distortion to the QOOO-cycle frequency current to control the amplification of the signal current. The distorted control current is impressed on thefilter 32, the resulting wave form being shown in Fig. 5 wherein the dips f of Fig. 4 are illustrated by a reduction in amplitude of the 9000-cycle control current. Since assaeue rectifier 58 and the biasing resistor It. By eliminating this filter, the system functions to restore the wave form of the original signal a.. It is to be noted that in the disclosures mentioned above such a filter is used and is essential to provide the volume control required. For instance, in Patent No. 2,029,389 the filter 3| is connected to the output of the rectifier, while in Patent No. 1,909,765,-filters 25 are employed. Inthe present invention, the gain of the amplifier 25 must be varied instantaneously or at a rate which is not in accordancewith the average value of the signal but is indirect relationship to variations in the wave form of the signal. Thus, the restoring action must take place immediately upon the signal wave form leaving the limits of the sound track area, the gain 01f the amplifier being in creased to maintain a rise in the signal current in proportion to the original signal shape, and

the gain of the amplifier and thus maintains the no-signal transmission current steady. Holes or scratches in the same modulated portions of the track function in the opposite direction, so as to cause no sound at the loudspeaker. In the case of variable density tracks, .the noise normally caused by dirt particles or scratches on any portion of the track will be eliminated since the control current modulations extend the full .wi dth of the track.

Another feature of the invention is its ability to restore abnormal high or low peaks totheir' normal value. For instance, should the light modulator incorrectly increase, or decrease certain peaks, the control current will likewise be, correspondingly increased and decreased incorrectly, which will vary the gain of the amplifier accordingly. Since the gain of the amplifier is this gain must be decreased in conformance with restore the eliminated peaks of the signal, but A will also maintain the average volume of the signal due to conditions causing a decrease thereof, such like.

As mentioned above, there is a preferred amplitude value for the pilot or control current, this value approximating the amount of overshooting anticipated. Thus, the amplitude of the control frequency, as set by the control It in Fig. 1, is ,such that it slightly exceeds the amount of the anticipated overshooting, and as long as a portion of a cycle of the control frequency is within the control track area, the overshot portion of the signal will be correctly restored upon reproduction.

Although the present invention has been described as being'embodied in a motion picture sound film recording and reproducing system, it is also applicable to disc or magnetic recording or other transmission systems inasmuch as the correction is accomplished electrically and is not dependent upon the type of sound record medium. Furthermore, the systemmay be used as a compressor or expander either by manually or automatically varying the amplitude of the 9000- cycle' control current at ll during recording an reproduction.

As mentioned above, the system will restore missing portions of wave forms in the same manner as described above with respect to overshooting. That is, should a scratch destroy a portion of the signal record, a portion of the control current record will also be eliminated, which varies the signal amplifier gain proportionally to the amount of record destroyed. In the case of large dirt particles on any portion of the film modulated by the control current, which normally produce plops at the loudspeaker, their efiect will be substantially eliminated as follows:

If we assume a dirt particle on the control cur-- rent modulated area during no signal, the dirt particle momentarily decreases the light to the cell and thusmomentarily decreases the steady value current in the transmission circuit. with the invention, the dirt particle also decreases the amplitude of the control current, which increases as a decrease in'lamp brilliancy or the varied in accordance with the instantaneous values of the control current which has-a higher frequency than any signal current modulation, the invention will accurately correct for this type of distortion.

The above-described invention, therefore, will provide substantially noiseless reproduction of dirty and scratched film, while eliminating distortion caused by overshot, mutilated and distorted sound tracks within reasonable limits.

What I claim as my invention is:

1. The method of correcting recorded signal distortion caused by the signal modulations overshooting the upper limit of a recording medium comprising simultaneously recording with a signal current a second independent current having a constant amplitude and frequency, said second current undergoing distortion at the points where said first signal overshoots said recording medium, reproducing said first signal current, and utilizing the distortion insaid second signal current for varying the amplification of said first signal current during reproduction thereof to restore the portions of said first signal not recorded at said points where said first signal overshot said recording medium.-

2 The method of correcting the distortion of recorded signals whose amplitude has exceeded the amplitude limit of a recording medium comprising simultaneousiy recording with said signal current a control current of constant amplitude and constant frequency, said control current undergoing distortion where said signal current modulations exceed the limit of said recording medium, reproducing said signal current and utilizingthe instantaneous variations in amplitude of said control current for correspondingly varying the amplification of said signal current at said points where said signal medium.

3. The method in accordance with claim 2 in which the amplitude of said control current exceeds the amplitude of overshooting by said signal current.

4. The method of restoring deleted portions of a signal record during reproduction of said record comprising varying the amplification of the reproduced signal record atthe points where said record portions are deleted in a direction and in an amount to-produce a. current corresponding to the deleted portions of said record.

5. The method of eliminating the eiiect of dirt and scratches on a sound record comprising generating a current corresponding to the modulations on a sound record, and maintaining said current in direct proportion to the amplitude of the modulations on said record by increasing and decreasing the amplification of said current in proportion -to the amount of increase and decrease in the generation of saidcurrent caused by said dirt and scratches on said record.

6. The method of controlling the reproduction of a 'sound signal record having recorded therewith a control record of constant amplitude and constant frequency comprisingsimultaneously translating said signal and control records into corresponding electrical currents, separating said currents, and varying the amplification of said signal current in accordance with the instantaneous values of said control current.

7. The method in accordance with claim 6 in which the frequency of the control record current is higher than the-highest signal frequenc of said signal record.

8. The method in accordance with claim 6 in which theamplification of said signal current is varied at a 'rate of twice that of the frequency of said control current.

9. A system of sound record reproduction comprising means for generating currents corresponding to a sound record, means for simultaneously generating currents corresponding to a control record, means for amplifying said currents, means for translating said signal record currents into sound waves, and means for, varying the amplification of said currents according to the instantaneous values of the currents of said control record.

10. A system of sound record reproduction in accordance with claim 9 in which the amplification is varied at a rate of twice that of the frequency of said control record currents.

11. Sound recording and reproduction apparatus comprising means. for recording a control current record of constant frequency and constant amplitude simultaneously with and in the same recordable area as said signal record,

ass-mus means for simultaneously generating currents corresponding to said signal record and said control record, means for segregating said control record currents from said signal record currents, a variable gain amplifier for said signal record currents, means for reproducing said sig- -nal record currents, and means for varying the gain of said amplifier for said signal record currents in accordance with the instantaneous amplitude values of said control record currents.

12. Sound recording and reproducing apparatus in accordance with claim 11 in which the gain of said amplifier for said signal record currents is varied at a rate of twice that of the frequency of said control record currents.

13. Sound recording and reproducing apparatus comprising means for simultaneously recording a control currentrecord of constant amplitude and constant frequency in the same area and combined with a signal record, means for reproducing said signal record, andmeans for varying the amplification of said signal record in accordance with the instantaneous values of said control record variations.

14. Asound reproducing system comprising a light sensitive device for producing currents corresponding to a combined signal and control record, a variable gain amplifier for saidcurrents, a sound reproducer, a filter for eliminating said control currents from said reproducer, a" rectifier, a filter for eliminating said signal currents from said rectifier, and means for impressing the output of said rectifier directly on said variable gain amplifier.

15. A sound system in accordance with claim 14 in which said first-mentioned filter is a 7000- cycle low-pass filter, said second filter is a 9000- cycle bandpass filter, and said rectifier is a fullwave rectifier.

FREDERICK G. ALBIN. 

